The few things that spring to mind:

Be polite but persistent

Procurement people at corporates are used to really pushy sales people; so if you barely make any effort you won't get through at all :)

Judge who you are talking too

Most people you talk to won't be able to give you a "yes", get past them as fast as you can (I usually just ask straight out to speak to their superior/boss etc.), they are there mostly as a filter. Also consider what sort of firm you're calling - in a 50+ person company it is likely to be a specific department who buy things (even if it is just an individual), below that the person probably has a different day-to-day job. I've had a lot of success, when talking to smaller firms, with the line "lets just drop all the corporate nonsense, we're both small firms and there is no need for me to make this the normal pain in the neck".

Forget all the marketing nonsense

You'll see a lot of "marketing best practices"; I mostly ignore them, they are designed for a company with a number of sales people to normalise the sales routines. You're hacking the sales routine, so take it naturally.

I particularly advise avoiding the template emails that you stick a name in front of... keep the templated stuff to short paragraphs in your emails & material you attach (i.e. PDF's) and try to write an honest "personal" note.

Honesty does get noticed BTW, it can set you apart from the usual inundation of marketing they get.

Be open to adaption, but have a firm limit

I once committed us to a normal sale but "with a few tweaks", it took me the best part of a month to get the tweaks in place. :) Accomodating companies requirements gets you a good name, but really consider if it is worthwhile, and be happy to say no.

Be prepared to walk away

You can't sell every time. And if you are desperate to sell then they can take advantage of it! Either by getting impractical modifications (see above point) or drumming down your price. Either that or they are just not interested, and you end up sounding desperate and silly.

Invest in CRM

Biggest piece of advice IMO; it is so so so so so easy to forget who you contacted & about what. It might take you weeks to find the right person to talk to at a firm, but in 6 months when you try to upsell them the new product you don't want to have to do all that work again :) Plus I had a silly incident a while back where I kept emailing the same introductory email to a poor CEO; I think he got it about 86 times before calling to ask me to stop :)

Call the CEO

Doesn't work all the time, but sometimes (particularly in smaller firms) you can get things moving (and seal a sale) by calling the CEO. It takes a bit of balls and a fast pitch (or social engineering) to stop them hanging up :)

My favourite line was "X suggested I call you about this product, Y, we're selling you so you can have an idea of how it will improve your business"

Where X is someone in the middle of the food chain who has, at some point, said "you need to talk to my boss". :)